WASHINGTON (CNN) -- Former Tennessee Sen. Fred Thompson has skidded into sixth place in a CNN/WMUR poll of likely Republican voters in New Hampshire, edged out by ex-Libertarian and anti-war Rep. Ron Paul and former Arkansas Gov. Mike Huckabee.

Fred Thompson has not been able to rally New Hampshire conservatives to his campaign.

Former Massachusetts Gov. Mitt Romney topped the poll, widening a lead he has held for months in neighboring New Hampshire, while Arizona Sen. John McCain and former New York Mayor Rudy Giuliani were running close in the second and third spots.

The CNN/WMUR poll was conducted by the University of New Hampshire between Wednesday and Sunday. Pollsters surveyed 404 Republican voters for the survey, which had a sampling error of plus or minus 5 percentage points.

Thompson came into the GOP race late with the hope of winning social conservatives unsatisfied with the rest of the party's field, and racked up a key endorsement from the National Right to Life Committee last week.

But the former star of the television drama "Law and Order" has trailed the GOP front-runners in early voting states since entering the race in August, and his support in New Hampshire dropped from 13 percent in a September poll to 4 percent in November's survey. Watch who's winning on key issues »

By contrast, Romney's support grew from 25 percent to 33 percent over the same period; McCain held steady at 18 percent; and Giuliani dipped from 24 to 16 percent.

Meanwhile, the percentage of support for Paul grew from 4 percent to 8 percent, putting him fourth among the GOP contenders in the Granite State.

Don't Miss

The Texas congressman, who once ran for president on the Libertarian Party ticket, has gained notice as the sole advocate of a U.S. withdrawal from Iraq among the GOP field -- and he raised eyebrows in Washington earlier this month when supporters said they raised $4.3 million in a single day of online fundraising. The figure can't be independently confirmed until Federal Election Commission reports are filed at the end of the year.

Among other contenders, Huckabee -- gaining ground ahead of the Iowa caucuses -- claimed 5 percent support in the poll. Colorado Rep. Tom Tancredo, who has made opposition to illegal immigration the centerpiece of his campaign, drew 1 percent; and California Rep. Duncan Hunter, the former chairman of the House Armed Services Committee, had the support of fewer than half of 1 percent.

Thompson's negatives have grown sharply since he entered the race.

In July, a CNN/WMUR poll of New Hampshire Republicans found 30 percent would not support him "under any circumstances" -- but in the new poll, that figure grew to 50 percent.

McCain's rating on the same question declined from 38 percent to 29 percent over the same period; Romney's showed a slight increase, from 17 percent to 19 percent; and the number of people who said they could not support Giuliani on any terms grew from 22 percent to 28 percent.

"New Hampshire Republicans are beginning to see a two-man race for the nomination: Romney versus Giuliani." said CNN senior political analyst Bill Schneider.

"Only 8 percent believe McCain is a likely national winner. That could be holding his numbers down in New Hampshire -- a state he has to win."

The war in Iraq remains GOP primary voters' top concern, though its importance has dropped since summer.

The issue is a major strength for McCain, a leading advocate of the 10-month-old U.S. campaign to pacify Baghdad and its surrounding provinces: Those polled rated him most capable of managing the four-year-old war by about a 3-1 margin when compared with Romney and Giuliani.

But the issue has faded since July, while likely voters' concerns about terrorism and illegal immigration have grown.

Those who ranked immigration the country's most important problem grew from 11 percent in July to 16 percent in November, while terrorism grew from 11 percent to 15 percent.

Giuliani's management of the aftermath of the September 11, 2001, attack on the World Trade Center has become a central selling point of his campaign, and GOP voters ranked him best-equipped to handle terrorism. Meanwhile, former businessman and 2002 Winter Olympics organizer Romney beat the other top contenders by better than 3-to-1 when it came to managing the economy, taxes and immigration.

But the poll suggests the GOP race remains fluid with about two months left before the New Hampshire primary, the official date of which remains unsettled. Only 14 percent of those surveyed said they had definitely decided on a candidate, while 29 percent said they were leaning toward one -- and 56 percent were still trying to decide.E-mail to a friend